1. Field of the Invention
This invention relates to the tempering of glass sheets and more specifically, to the rapid cooling of hot glass sheets immediately following their heating to a temperature sufficient for tempering while conveying the sheets through a cooling station immediately downstream of a furnace. In the furnace, the conveyed sheets may be supported on a gaseous bed with their major surfaces out of contact with solid members or on spaced, rotating conveyor rolls that make intermittent, momentary contact with the bottom glass sheet surface during said conveyance, or on a combination of rotating rolls and a gaseous support.
The cooling is accomplished using a system for supplying tempering medium in heat exchanging relationship and/or supporting relationship to a sheet or ribbon of glass. Most conveniently, the tempering medium comprises cold air blasted from a source of pressurized air.
The support system is particularly adapted for handling glass in sheet or ribbon form with minimum or no marring or otherwise producing uncontrollable deformation in the major surfaces with respect to the center of the thickness of the glass sheets. Tempering involves heating a glass sheet to an elevated temperature above its annealing range and then rapidly chilling its surfaces to below the strain point while the interior is still hot and continuing the rapid chilling until the entire glass sheet throughout its thickness cools to below its strain point. Such tempering causes the glass sheet to develop a skin of compression stress that surrounds the glass interior which is stressed in tension.
Such a stress distribution makes the tempered glass sheet much stronger than untempered glass so that tempered glass is less likely to fracture then untempered glass when struck by an object. Furthermore, in the less frequent times when an outside force is sufficiently large to cause tempered glass to fracture, tempered glass breaks up into a large number of relatively smoothly surfaced, relatively small particles which are far less dangerous than the relatively large pieces with relatively jagged edges that result from the fracture of untempered glass.
In fabricating glass through known manufacturing techniques of bending, tempering, annealing or coating and combinations of such techniques to form end products having characteristics and uses different from the original product, it is necessary to heat the glass sheets to a temperature above that at which the major surfaces or the contour thereof is changed by deforming stress on contact with solid members. Where it is desired to strengthen the glass, it is further necessary to cool the glass sheets rapidly from such a deformation temperature to a lower temperature below the annealing range of the glass. The effectiveness of such strengthening is improved by an increase in the rate at which heat is removed from the surfaces with respect to the center of the thickness of the glass sheets.
The final temper level in a glass sheet depends on the following variables:
1. Coefficient of thermal expansion of the glass in the viscosity range of 10.sup.10 to 10.sup.15 poises.
2. Relaxation characteristics of the glass in the viscosity range of 10.sup.10 to 10.sup.15 poises.
3. Heat conductivity and specific heat of the glass, including radiation characteristics.
4. Glass thickness.
5. Temperature distribution of the glass at instant cooling starts.
6. Time spent in various cooling stages.
7. Heat flux at the glass-tempering medium interface.
Heat flux involves both heat transfer coefficient and the temperatures of the glass and the medium.
For a given glass composition, equal heat extraction at the surface and equal temperatures, thicker glass sheets attain a higher temper than thin glass sheets.
The higher the glass temperature at the onset of cooling (up to a certain value), the higher the final residual stress attained.
The lower the apparent heat conductivity (which includes radiation) the higher is the stress attained.
Temper levels are higher with higher heat extraction rates at the surface.
Higher residual stresses are obtained with glasses having larger coefficients of thermal expansion in the viscosity range of 10.sup.10 to 10.sup.15 poises.
Higher tempers result (up to a certain limit) by increasing the duration of exposure to rapid cooling.
Optical defects become more severe when glass sheets have greater variability in local heat flux rates.
The "faster" the relaxation characteristics, the higher is the temper level attained.
Efficient glass sheet fabrication involving the techniques previously mentioned requires that the glass sheets undergoing treatment be conveyed while hot. The need to convey glass sheets at high temperature has involved undesirable deformation or marring of the major surfaces of glass sheets undergoing treatment due to physical contact of its major surfaces with supporting and conveying apparatus while the glass is at elevated temperatures. Glass sheets have been supported on spaced rotating rolls to reduce deformation due to their engagement with a solid member for more than momentary contact, and on gaseous beds to overcome more fully the defects of deformation and marring due to physical contact of their major surfaces with solid members at elevated temperatures.
Glass sheets have been conveyed through these gaseous beds by supporting the sheets at a small oblique angle to the horizontal and engaging the lower edges thereof with the peripheries of rotating driving discs arranged to form a path of driving along a common tangent. Alternatively, glass sheets have been conveyed through these gaseous beds by engaging the glass edge with fingers supported on carriages that move with the sheets through the entire extent of the gaseous beds to propel the glass sheets through a hot atmosphere within a furnace so as to heat the glass sheets to a temperature sufficient for tempering.
Attempts to cool the glass surfaces rapidly has involved the development of modules for supplying cool gas in a pressure pattern that is non-uniform across the dimension of the glass sheets transverse to their direction of movement through a space between opposite arrays of modules disposed above and below the upper and lower major surfaces of the conveyed glass sheets. Non-uniform rates of cooling have developed non-uniform stress patterns, which are accompanied by optical non-uniformities, sometimes called Q-lines.
One technique for minimizing the appearance of Q-lines has been the application of blasts of air through narrow elongated slots, preferably narrower than one millimeter, extending continuously across the entire width of the conveyed glass sheets. Recognizing that it is difficult to maintain uniform width along the entire length of narrow slots, the prior art used thin mesh screens to separate the walls of the narrow slots and to maintain the uniformity of slot width. The presence of screens impaired the free flow of air through the slots and, hence, limited the heat transfer rate due to the impingement against the glass surface by gas streams flowing through the narrow slots en route to the glass surface.
In one prior art modification, the entire set of prior art modules were made hollow to flow heat exchanging liquid through the hollow passages within the hollowed modules to improve the heat exchange rate by radiation. This solution introduced the problem of handling a liquid supply system for the entire cooling station of glass sheet. Leakage of cooling liquid occurred frequently and was difficult to eliminate. Such a large liquid supply system is needed for the entire cooling station that an alternative, less complicated system would be more desirable.
When glass must be tempered, a large escape area is usually provided for the impinging blasts of cooling medium, such as air, to be released readily from the central portion of the gaseous bed to avoid the establishment of a non-uniform pressure profile across the width of glass sheets transverse to the direction of glass movement. Such pressure profile increases toward the center of the glass and causes the glass to develop one of two metastable conditions, one in which the center of the glass sheets bows upward and another in which the center of the glass sheets bows downward.
When glass is supported on a gaseous support, the thickness of the gas bed is maintained as thin as possible to enable the incoming gas streams to impinge on the glass surface as efficiently as possible rather than blending with the gas bed that is already present. Therefore, when the glass develops a bowed shape due to the metastable conditions described previously, or when the glass develops a kink or departure from flatness, there is insufficient room for the glass to be conveyed between the upper and lower arrays of modules that supply the cool tempering medium needed to cool the glass sufficiently rapidly to develop a stress pattern through the glass thickness that strengthens the glass sufficiently so that the glass develops at least a partial temper.
Glass sheets tend to develop a kink, particularly in their leading edge, during a heat strengthening operation when a series of glass sheets are conveyed first through a hot atmosphere where the sheets in turn are heated to an elevated temperature sufficient for tempering and then through a cool atmosphere where the glass sheets are chilled at a rate sufficient to develop a stress pattern in the glass sheet. Monolithic glass sheets with a kink are difficult to install in architectural spandrels, are difficult to laminate to other glass sheets regardless of whether the other sheets are kinked, and are difficult to fabricate into multiple glazed insulating units comprising two or more glass sheets bonded to each other through spacing frames that extend around their peripheral edges only to form an insulating space between the glass sheets.
Glass sheets tempered with prior art apparatus having thin, elongated slots in modules for the application of cool gas under pressure developed kink, particularly in the first 12 inches (30 cm) of length. Such kinking sometimes interfered with the passage of the flat glass sheets between the upper and lower modules facing the path of glass sheet travel into the cooling area of the tempering apparatus. It was essential for the glass sheet to pass between the closely spaced modules disposed immediately downstream of the exit of the furnace at the entrance to the cooling station of tempering apparatus. It was also important to reduce said kinking sufficiently to enable heat strengthened glass sheets to be installed in architectural frames without developing stress inducing breakage and also to facilitate lamination and fabrication of multiple glazed units free of localized stress due to mismatch of glass sheets so fabricated.
In the past, the insulation of the upstream modules was attempted by inserting a body of heat insulating material, such as fused silica and fiber glass, between the furnace exit and the upstream walls of the upstream modules. However, it required a minimum thickness of 1 inch (25.4 mm) of heat insulating material to reduce module warpage to an acceptable level. Therefore, the upstream module had to be spaced more than 1 inch (25.4 mm) downstream of the furnace exit. Such a length of unsupported span of heat-softened glass required for the heat insulating material resulted in the treated glass sheets having a kink too large to be acceptable. The prior art required a more efficient heat insulation that would permit the placement of the upstream modules sufficiently close to the furnace exit to reduce kink to tolerable limits.
2. Description of Patents of Interest
U.S. Pat. No. 3,607,198 to Meunier et al. discloses a method and apparatus for moving hot glass sheets and similar ribbons that are supported pneumatically out of contact with solid surfaces by establishing alternate zones of static and kinetic gas pressure along the length of sheet movement. The Meunier et al. patent is designed for annealing glass sheets and utilizes slotted module housings with passages to impart blasts of cool air in the cooling portion of the apparatus through narrow slots to support and cool the glass. The modules throughout the length of the cooling station also contain hollow passages for cool liquid to flow therethrough to supplement the air cooling with radiation cooling. Maintaining a supply of cool liquid for each module in the cooling apparatus represents a logistics problem and a leakage problem that the glass cooling art would rather avoid.
The modules disclosed in the Meunier et al. patent are provided with passageways that are fed with water or other coolant material between each of the slots through which air is supplied to both support and cool the glass sheets. The cooling conduits extend throughout the entire length of the cooling area of the Meunier et al. apparatus and may be fed with any fluid such as water, air, or steam, which permits one to control the temperature of the gases flowing upward and of those in static state above the lower modules. Fluid is supplied to the fluid conducting conduits for the sole purpose of controlling the temperature of the air supplied to support and control the manner of glass cooling during an annealing process.
U.S. Pat. No. 4,046,543 to George B. Shields discloses apparatus for cooling and tempering glass sheets that comprises spaced slotted modules that face the opposite surfaces of a glass sheet throughout the length of a cooling station. Cold tempering medium is applied through oblique slots that direct air blasts obliquely away from a furnace exit in a downstream direction of glass sheet movement. The slots are formed in walls of longitudinally spaced, transversely extending modules and may be oriented to apply a transversely downward component of motion to the blasts of tempering medium that impinge against the opposite glass sheet surfaces to force the lower edges of the glass sheets against rotating discs that propel the glass sheets through the cooling station (or at least in the upstream portion of the cooling station).
U.S. Pat. No. 4,204,845 to George B. Shields and Eugene W. Starr discloses apparatus similar to that of the previous patent in which spaced slotted modules are provided in the upstream portion of the cooling station and so called "rosette" modules are provided in the downstream portion of the cooling station to supply a more diffuse pattern than the thin, discrete, laminated streams of tempering medium provided by the slotted modules in the upstream portion.
In both of these latter two patented apparatus, the pair of opposing upstream modules are exposed to the heat of the furnace that is radiated through the furnace exit. Therefore, the upstream modules, both above and below the path of travel taken by the glass sheets through the cooling area are likely to become warped by heat. Prior to the present invention, tempered glass sheets produced on such patented apparatus developed kinks during their production. The kinks were especially noticeable along the leading edge portion of the tempered glass sheets.